Method of making molds



F. SUDIA, JR

METHOD OF MAKING MOLDS Sept. 22, 1953 Filed Jan. 17, 1950 Fig.3.

INVENTOR Frank Sudio, Jr.

55 To Sewer Patented Sept. 22, 1953 METHOD OF MAKING MOLDS Frank Sudia,Jr., Turtle Creek, Pa., assgnor to Westinghouse Electric Corporation,East Pittsburgh, Pa., a corporation of Pennsylvania Application January17, 1950, Serial No. 139,094

3 Claims.

This invention relates to precision casting and in particular to themethod of making molds for use in precision casting. Y

Heretofore in the making of molds for precision casting, a slurry ofrefractory material and a binder has been invested about a wax patterndisposed in a suitable flask. The flask containing the pattern andinvestment material was thereafter subjected to a vibration treatment toeffect a packing of refractory material about the wax pattern afterwhich the binder was permitted to set or gel to effect the solidicationof the investment.

In forming the molds where refractory material is packed by vibration,it is found that there is a segregation 0f the fine particles of therefractory material in the upper end of the flask with the result thatitis necessary to cut olf the upper end of the flask and the moldmaterial therein and to discard such cut-off portion containing the finesegregation. It was thus impossible to obtain a uniform distribution ofthe refractory material or to control the particle size distributionwith the result that poor castings were obtained.

An object of this invention is to provide a method of making molds ofrefractory materials and a binder therefor in which a substantiallyuniform distribution of the refractory material is obtained.

Another object of this invention is to provide a method of making moldsof refractory materials and a binder therefor in which an excess of thebinder is removed and the refractory materials are so packed as to havea substantially uniform distribution in the resulting mold.

A further object of this invention is toutilize a differential ofpressure in making molds from refractory materials and a binder thereforto effect removal of a part of the binder and pack the refractorymaterials in a substantially uniform distribution to form the mold body.

Other objects of this invention will become apparent from the followingdescription when taken in conjunction with the accompanying drawing, inwhich:

Fig. 1 is a View in section of apparatus employed in practicing theteachings of this invention.

Fig. 2 is a View in section of apparatus employed in practicing anembodiment of this invention, and

Fig. 3 is a fragmentary View in section, and greatly enlarged, of a partof the apparatus of Figs. 1 and 2.

Referring to the drawing, this invention is illustrated by reference toapparatus utilized in practicing the method to be described. Thus, inFig. 1 there is illustrated a vacuum chamber Ill formed of the sidewalls I2, a base plate member I4 and a cover member I6, the side wallmembers I2 being connected through a valve I8 to a suitable vacuum pump20. The base plate member I4 is provided with a suitable drain pipe 22disposed to be connected through a Valve 24 to a sump (not shown), orsome other suitable disposal unit. As illustrated, the cover I6 isprovided with a central opening 2B, the purpose of which will beexplained more fully heref' inafter.

As illustrated, a filter 28 is disposed to seat on the cover I6, thefilter being formed of a perforated lter plate 30 and a layer 32 ofsuitable A cloth such as nylon, linen or muslin. In practice, the filterplate 30 is provided with a plurality of perforations 34 disposed in acentral area thereof corresponding to the opening 26 in the cover I6.The perforations 34 are preferably formed of 11g inch diameter holes on1A; inch centers. The layer 32 of filter cloth may be of any Suitablematerial, although muslin of the type known to the trade assixty-square, that is, 60 threads per square inch, has been found to bean exceptionally good filter medium. As illustrated, a rubber gasket 36is disposed between the lter plate 30 and the cover I6 in order. toobtain a good seal therebetween.

In practice, a wax pattern 38 is formed to the shape and size of thearticle which is to be later cast in metal. The wax of the pattern 38may be of any suitable formulation, reference being had to the waxdisclosed and claimed in Patent 2,467,796 which issued April 19, 1949,to I-I B.

`Willis as being an exceptionally good wax for this purpose. After thewax pattern 38 is formed, preferably with the base member 40 as a partthereof, it is mounted on a thin metal plate 42, better illustrated inthe enlarged view of Fig. 3, which is in the form of a disc having adiameter `substantially that of the diameter of the base 40. In mountingthe pattern 38 on the disc 42, the bottom of the base 40 is heatedsufficiently to soften the wax so that it will adhere to the one surfaceof the metal disc 42.

With the pattern 38 mounted on the metal disc 42 in this manner, the waxpattern 38 is dip-coated in a thin slurry of refractory material and asilicate binder to form a very thin coating (not shown) thereon whichwill have a smooth surface adjacent to the wax pattern and to makecertain that the surface of the wax is thorough- 1y covered by therefractory material and that all air is excluded therefrom. A suitabledipcoating is that disclosed in Patent 2,441,694, issued May 18, 1948,to Feagin et al. After the pattern 38 is thus immersed in the refractorycoating material, the dip coat is permitted to dry and the pattern 38 isthen positioned on the filter 28 as by means of a screw 44 which extendsthrough an opening in the filter plate 30 through a correspondingthreaded opening in the disc 42 upon which the base 40 is mounted.

With the pattern 38 assembled on the filter 28 in this manner, and thefilter 28 positioned on the vacuum chamber I as shown and described, anoperi end flask 46 is disposed about the pattern 38. The flask 46 may beof any suitable metal and is preferably formed of sheet steel having acoating of aluminum or aluminum phosphate on the inner surface thereofas disclosed and claimed in the copending application Serial No. 65,934,illed December 17, 1948, now abandoned, in the name of Gordon et al.With the open ended flask 46 thus encasing the dipcoated Wax pattern 38,a slurry 4B of refractory material and a binder therefor is admitted tothe flask 45 to completely fill it.

The slurry 48 may be of many different types of material and isgenerally formed of silica containing an accelerator such as MgO orNH4C1 and a binder formed of sodium silicate and a hydrolizing acid suchas hydrochloric acid 0r nitric acid. A particularly good formulation ofthe refractory solids is one formed of 49.6% of 80-mesh silica, of flinthaving a particle size ner than 200 mesh, 5.4% of MgO as an accelerator,and of a pre-shrunk 80-mesh refractory material. The slurry may containa varying amount of a binder although as a general rule about 26% of ahydrolized sodium silicate is employed with 64% of the refractorymaterials of the type just described.

It Will, of course, be understood that the refractory materials willvary depending upon the type of a mold desired and the mold cavity to beformed. Thus, for example, if buckling or separation of the dip-coatfrom the refractory body is likely to be obtained in the making of acasting in the resulting mold, it would oe desired to employ refractorymaterial which is not as fine as that described hereinbefore in makingthe mold. In such case, the refractory material of the slurry could beformed of 59.6% of 80- mesh silicate omitting the flint of the previousformulation. However, this invention is notV to be restricted to theformulations given as it will be understood that many other formulationscan be employed.

After the slurry has been admitted to the flask to the level of the topedge of the flask, it is noted that a thin lm. (not shown) of binder isformed on the upper surface of the slurry, which film constitutes anexcess of binder in the slurry. A vacuum is then applied to the chamberIU as by means of the vacuum pump 20, it being understood that the valve24 is closed and the valve I8 is opened, in order to remove the excessof binder from the slurry in the flask 4B. The differential of pressurecreated by applying the vacuum to the lower end of the flask causes thebinder to lter through the refractory material in the flask 46 andtherefore drain into the chamber I9. Under the influence of thedifferential pressure the refractory particles tend to pack while theexcess of the binder is being removed to lower the level of the slurryin the flask to a level indicated by the line 50. As soon as the fllm ofexcess binder disappears from the upper surface of the slurry and thelevel of the slurry is lowered to the indicated level 50, additionalslurry is added to the flask to bring the slurry to the top of the flask46, the vacuum being maintained. In practice it is found that the excessbinder of the slurry filling a flask having the dimensions of 7 inchesby 7' inches by 9 inches high is removed therefrom in a period of timeof about 20 minutes. The time, of course, will vary depending upon theamount of binder employed in the initial slurry, it being desired,however, that the differential pressure be applied for a sufficientlength of time to take the excess binder all of the Way through theinvestment and to leave only enough binder in the investment to wet theindividual particles of the refractory material.

In a particular example, utilizing the refractory formulation of -meshsilica and flint as given hereinbefore, and hydrolized sodium silicateas the binder, it was found that where 91A? pounds of binder wereemployed in the slurry to fill the flask 46 that the differentialpressure applied as indicated removed from 2 to 31/2 pounds of thebinder depending upon the length of time during which the differentialof pressure was applied.

After the excess binder has been removed from the refractory materialsin the flask 46, the vacuum is released and the flask is permitted tostand for a sufficient period of time, determined by the amount of theaccelerator in the refractory material, to effect a setting or gellingof the binder. This gellation of the binder will vary somewhat dependingupon the amount of accelerator therein, it being found that by using.27% MgO as the accelerator that the binder will set or gel inapproximately 13 minutes. When thus gelled, the mass in the flask 46 issubstantially solid except for the inclusion of a few air bubbles whichwill not harm the mold as they will not form a part of the surface ofthe mold cavity. The flask 46 can then be removed from the lter plateafter which the mold material is preferably aged for from 12 to 30 hoursin order to strengthen the gel and effect some air drying of the moldmateriaL Instead of the two step filling of the flask 46 with the slurry48 as described hereinbefore, a one step process may be employed inwhich case a longer flask 46 is used, being completely filled with theslurry and the vacuum is applied to remove the excess binder from theinvestment material. In such case, it is then necessary to cut off theextension of the flask 46 at the upper end thereof to the level of thegelled mass so as to give a flat surface for later seating the mold andreceiving the metal.

In either case, after the set or gelled mass within the flask 46 hasbeen permitted to age, the flask containing the refractory materials andthe wax pattern 38 is heated in the position illustrated as Icy-passingit between a bank of infra.- red lamps where the heating effects amelting of the wax of the pattern 38 and the wax flows from theinvestment material leaving a cavity therein. The dewaxing process will,of course, effect some drying of the refractory materials of the mold.The flask is then passed through a suitable furnace and is heated at atemperature of between 870 C. and 1000 C. for a period of time of abouttwo hours where the last traces of the wax are burned out of theinvestment material and the investment material is dried leaving acavity therein which is, in effect, a duplicate of the original waxpattern 38. The dried refractory material in the flask 46 is then readyto receive molten metal into the cavity formed in the material.

Instead of the vacuum packing method described with reference to theAapparatus of Fig. 1, the differential in pressure may be applied by apositive application of pressure to the top of the fiask 46 asillustrated in Fig. 2 of the drawing. In Fig. 2, like numerals areemployed for indicating identical parts with those of the apparatus ofFig. l, the main distinction being that insteadof the vacuum pump 20 ofFig. 1, a pressure pump 52 is employed to apply predetermined pressureto the slurry at the upper end of the flask 46. In this embodiment theconduits 54 and 56 may be open to the atmosphere, the conduit 54 actingas a vent whereas the conduit 56 functions as a drain.

In the positive pressure method, after the dipcoated wax pattern 36 ismounted on the lter 28 and positioned on the cover I6 of the chamber I6,as described with reference to the embodiment of Fig. 1, the flask isdisposed about the pattern 38 and slurry is admitted to the flask.Thereafter, a cover 58 having a groove 66 therein of the configurationIof the upper end of the flask 46 is seated on the flask 46. Preferably,a gasket 62 is disposed in the groove 60 of the cover 5B to receive theupper end of the iiask 46 and effect a good seal between the flask 46and the cover 58. The cover 58 is then clamped in the assembled positionon the flask 46 as by means of a plurality of C-clamps 64 disposed inspaced relation about the flask 46 and the pressure pump 52 is connectedthrough a valve 66 to an opening 68 in the cover 58. Positive fluidpressure is then applied to the slurry in the upper end of the flask 46to develop a differential in pressure of between 3 and 10 pounds persquare inch between the upper end of the flask and the lower end of theflask 46 to force the excess of the binder through the refractorymaterial of the investment and through the filter 28 into the chamberIll where it escapes only through the drain 56.

In practice, it is found that a pressure of less than 3 pounds persquare inch is insufficient for removing the excess of binder from themold material, with the result that it is impossible to obtainsufficient compacting of the fine particles of the refractory materialabout the wax pattern 38 to obtain the necessary strength in theresulting mold. On the other hand, however, pressure in excess of poundsper square inch cannot be employed satisfactorily because most of thepatterns 38 employed in the precision casting art have hollow spacestherein in order to reduce the Weight and save material thereof, andpressure in excess of 10 pounds will cause the collapse of suchpatterns.

Whether the differential of pressure is applied by the vacuum processdescribed with reference to Fig. 1 or by the positive pressure methoddescribed with reference to Fig. 2, it is found that the excess ofbinder can be removed in substantially the same time so long as thedifferential in pressure between the upper end of the flask and thelower end of the flask is between 3 to 10 pounds per 'square inch. Ineither case, the eX- cess of binder is removed without effecting asegregation of the ne particles of the refractory material so that auniform mold structure is obtained. In both cases it is necessary toemploy the dip-coated wax patterns as it is found that unless suchdip-coating is employed, the mold cavity formed in the resulting moldwill have rough spots and irregularities therein due to the entrapmentof air on the surface of the A Wax.

The method of this invention has distinct advantages over` the vibrationmethods practiced heretofore. In the vibration methods it is necessaryto control the liquid-solid ratio of the investment material as well asthe particle size distribution of the refractory materials thereof inorder to get packing of the refractory materials. With the differentialpressure method described hereinbefore, either the vacuum method or thepositive iiuid pressure method, the liquidsolid ratio is only criticalto the extent of having the investment slurry fluid enough to flow.Further, the particle size distribution may vary over a considerablerange without appreciable effect on packing. In the vibration method,the vibration amplitude is also critical and a constant vigilance mustbe kept on the operating conditions of the vibrators. However, with thedifferential pressure method, it is only necessary to maintain adifferential pressure suiiicient to provide filtering.

The differential pressure method described hereinbefore produces a moldin which there is no apparent segregation of the particles. With suchmolds it has been found possible to cast large articles having slightlycurved areas with a considerable saving in time, handling and materialover the practices employed prior to this invention. An additionaladvantage over the prior methods is that it is not necessary to seal thebase of the flask to the filter plate as the differential in pressureapplied to the slurry in the flask is sufficient for effecting anadequate seal between the bottom edge of the flask and the plate onwhich it is mounted.

While this invention has been described with reference to particularembodiments thereof, it will, of course, be understood that a number ofmodifications thereof are possible within the scope of the invention.Since the apparatus employed in practicing the method is of standardcomponents, it will be apparent that the method can be readilyreproduced by those skilled in the trade.

I claim as my invention:

1. In the method of making molds, the steps comprising, positioning awax pattern having a thin coating of refractory material thereover on afilter plate, disposing an open ended flask vertically about the coatedpattern on the filter plate, investing the coated pattern by introducinga slurry of refractory material and a hydrolized sodium silicate bindertherefor into the flask to cover the coated pattern, applying adifferential pressure between the ends of the flask to develop apressure at the upper end thereof that is between 3 and 10 pounds persquare inch higher than the pressure at the lower end of the flask todevelop a pressure at the upper end thereof that is higher than thepressure at the lower end of the flask to effect the filtering of theexcess of the binder therefrom through the filter plate and pack therefractory material about the coated pattern, effecting a gellation ofthe remaining binder within the flask, removing the wax of the patternfrom the solidified refractory material, and heating the solidifiedrefractory a material to harden the mass whereby a substantiallyy selid,mold is formed having a cavity there-4 in of the shape and sizev of theWax pattern.

2.y In the method of making molds, the steps GQmDKSrig, positioning aWax pattern having a thin coating ofinvestment material thereover on afilter plate, disposing an open ended flask vertically about the coatedpattern on the filter plate, introducing a slurry of investment materialcomprising finely divided refractory solids and a binder therefor intothe flask to a level to cover the coated pattern, applying adifferential of pressure to the ends of the flask to develop a pressureat the upper; end thereof that is 3 to 10 pounds per square inch higherthan the pressure at the lOwer end of the flask to cause excess of thebinder to flow through the finely divided refractory solids in thedirection of the pattern and to pass to the lower end of the llask andthereby to effect packing of the residual portion of the investment intoa homogeneous mass about the coated pattern, effecting gellation of theremaining binder Within the flask, and thereafter removing the waxpattern and heating the investment to form a substantially solid moldhaving a cavity of the shape and size of the pattern.

3, In the method of making molds, the steps comprising, positioning awax pattern having a thincoating of refractory material thereover on afilter plate, disposing an open ended flask vertcally about the coatedpattern on the lter plate, Y

lower end'of the flask by a value of from 3 to l0 pounds per square inchbelow the pressure at the upper end of the llask to thereby effect theremoval of the excess of binder from the invest? ment in the flask sothat the flow of the binder through the slurry compacts the silicaparticles about the Wax pattern and effects a substantially uniformdistribution and packing of the silica particles about the coatedpattern, effecting a gellation of the remaining binder of the rfrac:tory slurry in the flask, and thereafter removing the wax pattern andheating the bonded silica to form a substantially solid mold having acavity of the shape and size of the pattern.

FRANK sUDrA, JR.

References Cited in the file of this patent UNITED STATES PATENTS.

Number Name Date 1,742,721 Noch Jan. 7, 1930 1,921,350 Custer Aug. 8,1933 2,129,697 Louisot Sept. 13, 1938 2,434,780 Wiss et al Jan. 20, 19482,482,257 Frei, Jr Sept. 20, 1949 2,190,193 Barr Dec. 6, 1949 2,502,789James Apr. 4, 1950 FOREIGN PATENTS llnnber Country Date 6,751 GreatBritain of ,191,4 298,738 Switzerland Jan. 2, 1939 OTHER REFERENCES TheIron Age, November` 9, 1944, pp. 52-58.

